Thunderstorms found to squirt antimatter into space

Top NASA boffins analysing data from a gamma-ray telescope satellite in orbit above the Earth say they have discovered that thunderstorms, in addition to the various other things they do, emit "beams of antimatter" out of their tops.

"These signals are the first direct evidence that thunderstorms make antimatter particle beams," said Michael Briggs, a member of NASA's Gamma-ray Burst Monitor (GBM) team. Briggs was addressing a conference in Seattle this week.

The GBM whose readings Briggs and his colleagues have been poring over is mounted aboard the Fermi gamma-ray space telescope. Though primarily intended to peer deep into the universe to probe the various kinds of high-energy events which produce gamma rays (punchiest of all the various types of radiation to be found along the electromagnetic spectrum), Fermi has also proven handy for observing things on the Earth beneath it.

Specifically, Briggs and his chums have been using the gamma burst instrument to observe a phenomenon known as terrestrial gamma flashes (TGFs), brief bursts of intense radiation associated with lightning storms. Something on the order of 500 TGFs are thought to occur worldwide every day, but normally they pass undetected - Fermi has logged just 130 since being launched in 2008.

Quite apart from gamma ray blasts, already pretty high powered stuff, the Fermi boffins now believe that TGF events also cause thunderstorms to cough antimatter (in the form of positrons, anti-electrons) out of the upper atmosphere as well. In one particular case, a powerful thunderstorm over Zambia actually peppered the Fermi spacecraft with positrons despite the fact that Fermi was 2,800 miles away above Egypt at the time.

This might seem to be impossible, as the spacecraft's low orbit meant that it was shielded from the thunderstorm by the curvature of the Earth - the storm was well below its horizon. But the high-energy positrons belching from the top of the storm, being charged, were naturally affected by the Earth's magnetic field.

"Even though Fermi couldn't see the storm, the spacecraft nevertheless was magnetically connected to it," says Joseph Dwyer, another NASA gamma boffin. "The TGF produced high-speed electrons and positrons, which then rode up Earth's magnetic field to strike the spacecraft."

Rather than zapping out into space on a straight line like the gamma rays, the antimatter hurtled round the planet along the curving field lines and struck the orbiting Fermi. The positrons then annihilated themselves as they encountered normal matter within the satellite, and the entire mass involved was converted into more gamma rays: ones with the giveaway energy of 511,000 electron-volts, a sure sign that positrons have been annihilating themselves. These rays were of course sniffed at once by the GBM, much to the gratification of Briggs, Dwyer and their colleagues.

"The Fermi results put us a step closer to understanding how TGFs work," said Steven Cummer, also of the Fermi boffinry crew. "We still have to figure out what is special about these storms and the precise role lightning plays in the process."